The present invention generally relates to a constant-amplitude wave combination type amplifier, and more particularly to a constant-amplitude wave combination type amplifier based on an LINC (linear amplification with non-linear components) process.
Recently, constant-amplitude wave combination type amplifiers have been widely used in various communication devices, such as radio devices for use in mobile communications, multiple radio devices, satellite communication radio devices, and broadcasting devices. There is a need to realize such constant-amplitude wave combination amplifiers which consume a small amount of power and which are configured of a small number of structural elements.
Constant-amplitude wave combination type amplifiers based on the LINC process are known as amplifiers which meet the above-mentioned need, and are disclosed in, for example, D. C. Cox, "Linear Amplification with Non-linear Components", IEEE Transactions on Communications, December 1974, pp. 1942-1945. An application of constant-amplitude wave combination type amplifiers based on the LINC process is proposed in Japanese Laid-Open Patent Application No. 1-284106.
FIG. 1 shows a conventional constant-amplitude wave combination type amplifier. As shown, the amplifier has a constant-amplitude wave operation circuit 61, two amplifiers 62 and 63, and a wave combining circuit 64. The constant-amplitude wave operation circuit 61 is formed of a digital signal processing circuit, and calculates an orthogonal wave Y orthogonal to an input wave X so that each combined wave obtained by combining the waves X and Y with each other has a constant amplitude. Then, the constant-amplitude wave operation circuit 61 outputs constant-amplitude waves A and B which have constant envelopes having equal amplitudes by the above-mentioned combination of the waves X and Y. Capital letters X, Y, A and B are vector amounts which include phases.
The amplifiers 62 and 63, which are formed of non-linear amplifiers such as C-class amplifiers, amplify the constant-amplitude waves A and B, respectively. The wave combining circuit 64 combines constant amplitude waves kA and kB respectively output by the amplifiers 62 and 63, and generates an output wave kX which is an amplified version of the input wave X.
The conventional configuration shown in FIG. 1 has an advantage in that the amplifiers 62 and 63 are not required to be formed of linear amplifiers, since it is sufficient to amplify the constant-amplitude waves A and B of the constant envelopes. In addition, since the original signal can be reproduced by the wave combining circuit 64, it is possible to linearly amplify the input wave X having little distortion irrespective of the use of the non-linear amplifiers.
As has been described previously, the constant-amplitude wave operation circuit 61 is formed of a digital signal processing circuit. However, the digital signal processing circuit consumes a large amount of power and is not suitable for high-speed signal processing. Further, the use of the digital signal processing circuit causes an increase in the size of the amplifier. For the above reasons, the use of conventional constant-amplitude wave combination type amplifiers is limited to special applications in which the amount of power consumed in the digital signal processing circuit or the size of the amplifier are negligible, or applications in which data is transferred at a low speed.